A WikiWord is, itself, a WikiWord, that is to say it's a word with mixed capitalization in the best CamelCase tradition. Most frequently, these are created by concatenating two or more capitalized words together, without a space between them. Just any capitalization won't do it though; specifically, you must have at least two capital letters each followed by at least one lower-case letter, so capitalized acronyms and abbreviations by themselves won't trigger the auto-linking magic.
The "wiki" of the word comes from the fact that, if you enter something LikeThis into the text on most wiki pages, it’ll automatically convert it into a link to the page going by the same name. Most wikis will also create links automatically when you surround them with brackets, much like the protocol used in a certain other massively-contributed knowledge repository kind of site.
Note #1: At least on the wiki package that I use for my own purposes (PHPwiki), the page created from a WikiWord will have the name, and respective URL, of the WikiWord, but the title of the page created from it will have spaces between the individual components of the term. This is as opposed to a term containing spaces, which will have ugly %20s where the spaces should be.
Note #2: WikiWords are depreciated on some wikis, most notably Wikipedia, which no longer automatically converts such terms into hyperlinks.